Illumination system comprising a radiation source and a fluorescent material

ABSTRACT

The invention is concerned with an illumination system, comprising a radiation source and a fluorescent material comprising at least one phosphor capable of absorbing a part of light emitted by the radiation source and emitting light of wavelength different from that of the absorbed light; wherein said at least one phosphor is an oxido-nitrido-silicate of general formula EA 2-z Si 5-a B a N 8-a O a :Ln z , wherein 0&lt;z≦1 and 0&lt;a&lt;5; comprising at least one element EA selected from the group consisting of Mg, Ca, Sr, Ba and Zn and at least one element B selected from the group consisting of Al, Ga and In, and being activated by a lanthanide selected from the group consisting of cerium, europium, terbium, praseodymium and mixtures thereof. The invention is also concerned with a phosphor, which is an oxido-nitrido-silicate of the general formula EA 2-z Si 5-a B a N 8-a O a :Ln z , wherein 0&lt;z≦1 and 0&lt;a&lt;5; comprising at least one element EA selected from the group consisting of Mg, Ca, Sr, Ba and Zn and at least one element B selected from the group consisting of Al, Ga and In, and being activated by a lanthanide selected from the group consisting of cerium, europium, terbium and mixtures thereof.

The present invention generally relates to an illumination systemcomprising a radiation source and a fluorescent material comprising aphosphor. The invention also relates to a phosphor for use in suchillumination system.

More particularly, the invention relates to an illumination system andfluorescent material comprising a phosphor for the generation ofspecific, colored light, including white light, by luminescent downconversion and additive color mixing based on a ultraviolet or blueradiation emitting radiation source. A light-emitting diode as aradiation source is especially contemplated.

Recently, various attempts have been made to make white light emittingillumination systems by using light emitting diodes as radiationsources. When generating white light with an arrangement of red, greenand blue light emitting diodes, there has been such a problem that whitelight of the desired tone cannot be generated due to variations in thetone, luminance and other factors of the light emitting diodes.

In order to solve these problems, there have been previously developedvarious illumination systems, which convert the color of light, which isemitted by light emitting diodes, by means of a fluorescent materialcomprising a phosphor to provide a visible white light illumination.

U.S. Pat. No. 5 998 925 discloses a white light emitting LED device. Ituses yttrium aluminum garnet doped with cerium, Y3Al5O12:Ce, to convertblue emission of an InGaN-diode into yellow to produce white light ofsuitable color temperature. Another approach according to WO 00/33390uses a combination of a blue emitting LED together with a green and redphosphor. The phosphors are at least one of a first phosphor, amongothers thiogallates (Sr,Ca,Ba)(Al,Ga)2S4:Eu, and at least one of asecond phosphor, among others metal sulfide SrS:Eu, or (Ca,Sr)S:Eu, orthiogallate CaLa2S4:Ce, to produce white light of distinct colortemperature. The thiogallate (SrCa,Ba)(Al,Ga)2S4:Eu can be used togenerate specific colors together with light emitting elements such asblue LEDs.

The above mentioned phosphors can be used to produce white light withvarious color temperatures and suitable index of color rendering byluminescent down conversion of primary LED emission, but they exhibitseveral drawbacks related to total conversion efficiency, absorptionstrength, emission wavelength tunability, thermal quenchingcharacteristics and life time which are of high importance for usingthese phosphors in LEDs.

Especially when a light emitting diode having a high-energy band gap isused to improve the conversion efficiency of the phosphor material, thenenergy of light emitted by the semiconductor is increased. The number ofphotons having energies above a threshold, which can be absorbed by thephosphor material, increases, resulting in more light being absorbed andthe efficiency being increased. But also the energy absorbed by thefluorescent material inevitably increases, resulting in more significantdegradation of the fluorescent material. Use of the light emitting diodewith higher intensity of light emission for an extended period of timecauses further more significant degradation of the fluorescent material.

Also the fluorescent material provided in the vicinity of thelight-emitting component may be exposed to a high temperature such asrising temperature of the light emitting component and heat transmittedfrom the external environment.

Further, some fluorescent materials are subject to accelerateddeterioration due to combination of moisture entered from the outside orintroduced during the production process, the light and heat transmittedfrom the light emitting diode.

By US 2002/0043926 A1 there is provided a light-emitting unit whichcomprises:

a light-emitting device for emitting light with a wavelength range offrom 360 nm to 550 nm; and a fluorescent material made of Ca—AI—SiO—Noxynitride glass activated with Eu²⁺; wherein a part of light emittedfrom the light emitting device is emitted outward after it is subjectedto wavelength conversion by the fluorescent material.

Still there is an ongoing need to generate new phosphor compositions toimprove efficiency and color quality in luminescent devices,particularly in the production of white light.

Thus the present invention provides an illumination system, comprising aradiation source and a fluorescent material comprising at least onephosphor capable of absorbing a part of light emitted by the radiationsource and emitting light of wavelength different from that of theabsorbed light; wherein said at least one phosphor is anoxido-nitrido-silicate of general formulaEA_(2−z)Si_(5−a)B_(a)N_(8−a)O_(a):Ln_(z), wherein 0<z≦1 and 0<a<5.

comprising at least one element EA selected from the group consisting ofMg, Ca, Sr, Ba and Zn and at least one element B selected from the groupconsisting of Al, Ga and In, and, being activated with a lanthanideselected from the group consisting of cerium, europium, terbium,praseodymium and mixtures thereof.

This type of phosphor emits in the red spectral range of the visiblespectrum and thus can provide the red component in LEDs emittingspecific colors or white light. Total conversion efficiency can be up to90%. Additional important characteristics of the phosphors include 1)resistance to thermal quenching of luminescence at typical deviceoperating temperatures (e.g. 80° C.); 2) lack of interfering reactivitywith the encapsulating resins used in the device fabrication; 3)suitable absorptive profiles to minimize dead absorption within thevisible spectrun; 4) a temporally stable luminous output over theoperating lifetime of the device and; 5) compositionally controlledtuning of the phosphors excitation and emission properties.

Another aspect of the present invention provides an illumination systemwherein the

fluorescent material comprises a phosphor of general formulaEA_(2−z)Si_(5−a)B_(a)N_(8−a)O_(a):Ln_(z), wherein 0<z≦1 and 0<a<5comprising at least one element EA selected from the group consisting ofMg, Ca, Sr, Ba and Zn and at least one element B selected from the groupconsisting of Al, Ga and In, and being activated with an lanthanideselected from the group consisting of cerium, europium, terbium andmixtures thereof and a yellow or green phosphor.

Preferably the yellow or green phosphor is selected of the group ofMS:Eu,Ce,Cu comprising at least one element selected from the groupM=Mg, Ca, Sr, and Zn;

MN₂S₄:Eu,Ce comprising of at least one element selected from the groupM=Mg, Ca, Sr, and Zn at least one element selected from the group N=Al,Ga, In, Y, La, Gd,

(Re_(1−r)Sm_(r))₃(Al_(1−s)Ga_(s))₅O₁₂:Ce, where 0≦r<1 and 0≦s≦1 and Reselected from Y, Lu, Sc, La and Gd

and (Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and0<z<1

The emission spectrum of such a fluorescent material has the appropriatewavelengths to obtain together with the blue light of the LED a highquality white light with good color rendering at the required colortemperature.

Preferably the fluorescent material of the illumination system comprisesa phosphor of general formula(Sr_(1−x)EA_(x))_(2−z)Si_(5−a)(Al_(1−b)B_(b))_(a)N_(8−a)O_(a):Ln_(z),wherein 0<a<5, 0<b≦1, 0<x≦1 and 0<z≦1

Especially preferred are Eu(II) activated oxido-nitrido-silicatecomprising an element EA selected from the earth alkaline metals Sralone or in combination with Ca and Ba and metal B selected as Alaccording to general formula(Sr_(1-x-y)Ba_(x)Ca_(y))_(2−z)Si_(5−a)Al_(a)N_(8−a)O_(a):Eu_(z) wherein0<a<5, 0<x≦1, 0≦y≦1. and 0<z≦1.

In particular, the invention relates to specific phosphor compositionSr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04) which exhibit a high quantum efficiency of80-90%, high absorbance in the range from 370 nm to 470 nm of 60-80% andlow loss, below 10%, of the luminescent lumen output from roomtemperature to 100° C. due to thermal quenching.

The invention is also concerned with a phosphor capable of absorbing apart of light emitted by the radiation source and emitting light ofwavelength different from that of the absorbed light; wherein said atleast one phosphor is an oxido-nitrido-silicate of general formulaEA_(2−z)Si_(5−a)B_(a)N_(8−a)O_(a):Ln_(z), wherein 0<z≦1 and 0<a<5.

comprising at least one element EA selected from the group consisting ofMg, Ca, Sr, Ba and Zn and at least one element B selected from the groupconsisting of Al, Ga and In, and being activated with a lanthanideselected from the group consisting of cerium, europium, terbium andmixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a tri-color white LED lamp comprising atwo-phosphor mixture of Sr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04) and Sr Ga₂S₄:Eupositioned in a pathway of light emitted by an LED structure.

FIG. 2 discloses the X-ray diffraction diagram ofSr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04).

FIG. 3 discloses an emission spectrum of Sr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04)upon excitation by a blue LED at 460 nm.

FIG. 4 discloses an excitation spectrum of Sr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04)upon excitation by a blue LED at 460 nm.

DETAILED DESCRIPTION

The present invention focuses on a lanthanide-activatedoxido-nitrido-silicate as a phosphor in any configuration of anillumination system containing a radiation source, including, but notlimited to discharge lamps, fluorescent lamps, LEDs, and LDs. While theuse of the present phosphor is contemplated for a wide array oflighting, the present invention is described with particular referenceto and finds particular application to LEDs. As used herein, the term“light” encompasses radiation in the UV, IR and visible regions of theelectromagnetic spectrum.

The fluorescent material according to the invention comprises as aphosphor a

lanthanide activated oxido-nitrido-silicate comprising an element EAselected from the earth alkaline metals Mg, Ca, Sr, Ba and from zinc andat least one additional metal B selected from the group of Al, Ga andIn.

The phosphor conforms to the general formulaEA_(2−z)Si_(5−a)B_(a)N_(8−a)O_(a):Ln_(z), wherein 0<z≦1 and 0<a<5.

The term “phosphor” is used throughout this specification and theappended claims in its conventional meaning, to mean a luminescentmaterial that can absorb an excitation energy (usually radiation energy)and store this energy for a period of time. The stored energy is thenemitted as radiation of a different energy than the initial excitationenergy. For example, “down-conversion” refers to a situation where theemitted radiation has less quantum energy than the initial excitationradiation. Thus, the energy wavelength effectively increases, and thisincrease is termed a “Stokes shift”. “Up-conversion” refers to asituation where the emitted radiation has greater quantum energy thanthe excitation radiation (“Anti-Stokes shift”).

The term “activator” is used herein to mean a substance incorporated ina phosphor as an activator or luminescent center, eithersubstitutionally or interstitially with respect to the crystal latticeof the host substance, or even adsorbed on a surface of the crystallattice of the host substance. The term “activator” can also includeco-activators used, for example, to facilitate energy transfer.

The activators play a decisive role due to the composition dependentcrystal field dependence of absorption, emission and conversion behaviorof the optical 4f-5d transitions of the emitting centers.

The lanthanide activator of the present invention is chosen from thegroup consisting of cerium, europium, terbium, praseodymium and mixturesthereof. The preferred lanthanide, or mixtures thereof, is chosen tocorrespond to the emission wavelengths desired in the light-emittingdevice.

The choice of the lanthanide activator of the phosphor determines theemission wavelength of the phosphor. For example, when the excitingradiation is in the UV range and lanthanide Ln is europium(II), the peakin the emission spectrum is typically of longer wavelength (580-660 nm),and appears red. When the exciting radiation is in the UV range andlanthanide is terbium (III), the peak in the emission spectrum istypically of shorter wavelength, and appears green. When the excitingradiation is in the UV range and the lanthanide is cerium (III), thepeak in the emission spectrum may be of even shorter wavelength, andappear blue. It is therefore apparent that by choosing the correctlanthanide component in the present inventive phosphor, UV radiationfrom a LED can be converted to different visible colors.

The physical properties of the phosphor, such as the location and shapeof the absorption spectrum can also be manipulated by the choice of theEA metals, their relative amounts and the amount of B metals and theirrelative amount

The metal EA is chosen from the group including, but not limited to Mg,Ca, Sr, Ba and Zn.

The physical properties of the present phosphor may also be varied bythe choice of the amount of substitution of silicon by metal B in thehost lattice. The metal B is chosen from the group including, but notlimited to Al, Ga and In.

In addition, silicon may alternatively be substituted by germanium.

The incorporation of oxygen into the nitridosilicate lattice of theknown phosphors such as EA₂Si₅N₈:Eu decreases the proportion of covalentbond and ligand field splitting with respect to the activator cation. Asa consequence this leads to a shift of excitation and emission bands toshorter wavelengths in comparison to nitridosilicate lattices.

Preferred examples of this class of phosphors are(Sr _(1−x) EA _(x))_(2−z) Si _(5−a)(Al _(1−b) B _(b))_(a) N _(8−a) O_(a):Ln _(z),

wherein 0<a<5, 0<b≦1, 0<x≦1 and 0<z≦1

Especially preferred are Eu(II) activated oxido-nitrido-silicatecomprising an element EA selected from the earth alkaline metals Sralone or in combination with Ca and Ba and metal B selected as Alaccording to general formula(Sr_(1-x-y)Ba_(x)Ca_(y))_(2−z)Si_(5−a)Al_(a)N_(8−a)O_(a):Eu_(z) wherein0<a<5, 0<x≦1, 0≦y≦1 and 0<z≦1.

In particular, the invention relates to specific phosphor compositionSr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04) which exhibit a high quantum efficiency of80-90%, high absorbance in the range from 370 nm to 470 nm of 60-80% andlow loss, below 10%, of the luminescent lumen output from roomtemperature to 100° C. due to thermal quenching.

Sr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04) emits in the red spectral range of thevisible spectrum and thus can provide the red component in LEDs emittingspecific colors or even white light.

Many other examples of suitable phosphors which adhere to the generalformula EA_(2−z)Si_(5−a)B_(a)N_(8−a)O_(a):Ln_(z), wherein 0<z≦1 and0<a<5 are contemplated and will be obvious to the skilled artisan.

The above list is intended to be illustrative and should not beconstrued to be limiting in any way.

These red to yellow-red emitting phosphors are prepared by the followingtechnique: To prepare the mixed oxides high purity nitrates, carbonates,oxalates and acetates of the earth alkaline metals or zinc and thelanthanides were dissolved with stirring in 25-30 ml deionized water.The solutions are stirred with heating on a hot-plate until the waterhas evaporated resulting in a white or yellow paste, depending oncomposition.

The solids are dried overnight (12 hours) at 120° C. The resulting solidis finely ground and placed into a high purity alumina crucible. Thecrucibles are loaded into a charcoal-containing basin and then into atube furnace and purged with flowing nitrogen/hydrogen for severalhours. The furnace parameters are 10° C./min to 1300° C., followed by a4 hour dwell at 1300° C. after which the furnace is turned off andallowed to cool to room temperature.

These metal oxides are mixed with silicon nitride Si₃N₄ and the nitridesof the B-Metals such as AlN in predetermined ratios.

The powder mixture is placed into a high purity alumina crucible. Thecrucibles are loaded into a charcoal-containing basin and then into atube furnace and purged with flowing nitrogen/hydrogen for severalhours. The furnace parameters are 10° C./min to 1600° C., followed by a4 hour dwell at 1600° C. after which the furnace is slowly cooled toroom temperature.

The samples are once again finely ground before a second annealing stepat 1600° C. is performed.

Luminous output may be improved through an additional third anneal atslightly lower temperatures in flowing argon.

The phosphor prepared by such method according to the general formulaEA₂Si₃Al₂N₆O₂:Eu comprises a host lattice with the main components ofSi, N and Al. It may also comprise traces of F, Cl, H, C and O. The hostlattice has a structure consisting of (N—Si—N—Al—N)-units in athree-dimensional network, wherein silicon as well as aluminum aretetrahedrically surrounded by nitrogen and oxygen.

Within the three dimensional network alkaline earth ions such ascalcium, strontium, barium, magnesium and zinc as well as europium(II)are incorporated.

X-ray diffraction of Sr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04) as shown in FIG. 2 isconsistent with the x-ray diffraction of Sr₂Si₅N₈ [T. Slieper, W.Milius, W. Schnick, Z. Anorg. Allg. Chem. 621 (1995) 1380] with certainsmall deviations of position and intensity due to the substitution ofdivalent metal ions and europium for strontium, aluminum for silicon andoxygen for nitrogen.

In an exemplary embodiment, the physical characteristics of the redphosphor Sr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04) are described by FIGS. 2 and 3.FIG. 2 is the excitation spectrum of the noted red phosphor and is ameasurement of the relative intensity of the red emission versusexcitation wavelength, while the red emission is measured at a constantwavelength. FIG. 3 is the emission spectra of the noted red phosphor andis a measurement of the relative intensity of emitted light at variouswavelengths while the excitation is held constant.

In FIG. 3 it can be seen that when the emission intensity of thephosphor at 615 nm is measured, it is most intense when excited byradiation in the range of about 300-450 nm, which is in the near-UV toblue range of the electromagnetic radiation spectrum. FIG. 2 shows thatwhen the excitation of the phosphor is held constant at 450 nm (thesample is excited only by radiation at 450 nm), the emission wavelengthof the phosphor is strongest in the range of about 605-630 nm, which isperceived as red by the eye.

The invention also concerns an illumination system comprising aradiation source and a fluorescent material comprising at least onephosphor of general formula EA_(2−z)Si_(5−a)B_(a)N_(8−a)O_(a):Ln_(z)with addition of other well-known phosphors, which can be combined toachieve a specific color or white light when irradiated by a LEDemitting primary UV or blue light as specified above.

Accordingly, the phosphors may be mixed or blended to produce desiredcolors. It is generally known that phosphor powders do not interact as aresult of lamp maling and they exhibit the beneficial property thattheir spectra are cumulative in nature. Hence, the spectrum of anillumination system that includes a blend of phosphors will be a linearcombination of the spectra of the radiation source and the individualphosphors. For example, if a red phosphor of the present invention,Sr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04) were mixed with any blue phosphor thelight produced would appear purple to the eye.

Exemplary embodiments of the present invention include an UV- or blueLED and a fluorescent material comprising at least two phosphors whichtogether produce white light having pleasing characteristics. Preferablygreen and red phosphors are selected so that they are excited by theblue-emitting LED. The red phosphor is chosen to beSr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04)

The green to yellow color-emitting phosphor typically has an emissionpeak at 510-560 nm with a full width at half maximum of not more than60.

Said at least one other green to yellow color-emitting phosphor maycomprise a photoluminescent metal sulfide MS comprising at least oneelement selected from the group M=Ba, Mg, and Zn alone or in combinationwith at least one of Sr, Ca; the sulfide being activated with europium,cerium or copper.

Said at least one other phosphor may also comprise at least one greenphosphor comprising a complex metal thiometallate photoluminescentmaterial MN2S4 comprised of at least one element selected from the groupM=Mg, Zn, alone or in combination with at least one of Ba, Sr, Ca, andat least one element selected from the group N=Al, Ga, alone or incombination with In, Y, La, Gd, the thiogallate being activated with atleast one of europium (Eu) and cerium (Ce).

Said at least one phosphor may also comprise at least one green phosphorcomprising garnet fluorescent material represented by a general formula(Re_(1−r)Sm_(r))₃(Al_(1−s)Ga_(s))₅O₁₂:Ce, where 0≦r<1 and 0≦s≦1 and Reis at least one selected from Y, Lu, Sc, La and Gd, activated withcerium.

Said at least one phosphor may also comprise at least one green phosphorcomprising silicate fluorescent material represented by a generalformula (Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and0<z<1, activated by europium.

Radiation sources suited to use in the present invention include but arenot limited to GaN-based (InAlGaN) semiconductor devices. Suitable GaNsemiconductor materials for forming the light emitting components aregenerally represented by the general formula In_(I)Ga_(j)Al_(k)N, whereI, J, and K are greater than or equal to zero, and I+J+K=1. The nitridesemiconductor materials may thus include materials such as InGaN andGaN, and may be doped with various impurities, for example, forimproving the intensity or adjusting the color of the emitted light.

While the invention is described with particular reference to UV/bluelight emitting components, it should be appreciated that light emittingcomponents which emit light of a different region in the electromagneticspectrum may also be used. For example, a red-emitting LED or LD, suchas an aluminum indium gallium phosphide (AlInGaP) LED would also beapplicable.

Electroluminescent radiation sources include semiconductor opticalradiation emitters and other devices that emit optical radiation inresponse to electrical excitation. Semiconductor optical radiationemitters include light emitting diode LED chips, light emitting polymers(LEPs), organic light emitting devices (OLEDs), polymer light emittingdevices (PLEDs), etc.

Moreover, light emitting components such as those found in dischargelamps and fluorescent lamps, such as mercury low and high pressuredischarge lamps, sulfur discharge lamps, and discharge lamps based anmolecular radiators are also contemplated for use as radiation sourceswith the present inventive phosphor compositions.

Any configuration of an illumination system which includes a lightemitting diode as a radiation source and a phosphor composition iscontemplated in the present invention. In an exemplary embodiment, thephosphor is located adjacent to the LED. In another embodiment, thephosphor is situated between encapsulant layers and is not in directcontact with the LED. In yet another embodiment, the phosphor isdispersed throughout an encapsulating layer. Notwithstanding thesedescribed configurations, the skilled artisan will recognize that anyLED configuration may be improved by the inclusion of the presentinventive phosphor.

A detailed construction of such light-emitting device is shown in FIG.1.

FIG. 1 shows a schematic view of the device of the present invention.The device comprises LED 1. LED 1 is positioned in a reflector cup 2.LED 1 emits light in a pattern. A phosphor composition 4,5 is positionedin the pattern. The phosphor composition is embedded in a resin 3. Inthis example, reflector cup 2 can modify light pattern if light isreflected into a space not previously covered by the initial lightpattern (e.g. in the case of a parabolic reflector). It is understoodthat one of ordinary skill in the art can provide reflector cup 2 in anyshape that optimizes reflection of light back to phosphor composition4,5, or optimizes positioning of LED 1 to provide a light pattern forefficient conversion. For example, the walls of reflector cup 2 can beparabolic.

In one embodiment, the device further comprises a polymer forencapsulating the phosphor or phosphor blend. In this embodiment, thephosphor or phosphor blend should exhibit high stability properties inthe encapsulant. Preferably, the polymer is optically clear to preventsignificant light scattering. In one embodiment, the polymer is selectedfrom the group consisting of epoxy and silicone resins. A variety ofpolymers are known in the LED industry for making 5 mm LED lamps. Addingthe phosphor mixture to a liquid that is a polymer precursor can performencapsulation. For example, the phosphor mixture can be a powder.Introducing phosphor particles into polymer precursor liquid results information of a slurry (i.e. a suspension of particles). Uponpolymerization, the phosphor mixture is fixed rigidly in place by theencapsulation. In one embodiment, both the composition and the LED areencapsulated in the polymer.

The use of a phosphor capable of absorbing a part of light emitted bythe radiation source and emitting light of wavelength different fromthat of the absorbed light; wherein said at least one phosphor is anoxido-nitrido-silicate of general formulaEA_(2−z)Si_(5−a)B_(a)N_(8−a)O_(a):Ln_(z), wherein 0<z≦1 and 0<a<5

comprising at least one element EA selected from the group consisting ofMg, Ca, Sr, Ba and Zn and at least one element B selected from the groupconsisting of Al, Ga and In, and being activated with a lanthanideselected from the group consisting of cerium, europium, terbium,praseodymium and mixtures thereof is especially advantageous if thephosphor composition is applied as a thin film or in a small volume,because they are not sensitive to higher temperatures, which result inthin layers due to heat generated by the Stokes shift together withstrong absorption and consequently very small light penetration depth.

The phosphor comprising fluorescent material may be fabricated byeventually dry blending phosphors in a suitable blender and then assignto a liquid suspension medium or the individual phosphor or phosphorsmay be added to a liquid suspension, such as the nitrocellulose/butylacetate binder and solvent solution used in commercial lacquers. Manyother liquids including water with a suitable dispersant and thickeneror binder such as polyethylene oxide can be used. The phosphorcontaining composition is painted or coated or otherwise applied on theLED and dried.

Otherwise the phosphor or phosphors can be combined with a suitablepolymer system, such as polypropylene, polycarbonate, orpolytetrafluoroethylene, to a phosphor composition, which is then coatedor applied to the LED and dried, solidifies, hardeners, or cured. Theliquid polymer system may optionally be UV cured or room temperaturecured to minimize any heat damage to the LED.

Otherwise a clear polymer lens made of suitable plastic such aspolycarbonate or other rigid transparent plastic is molded over the LED.Lens may be further coated with anti-reflective layers to facilitatelight to escape the device.

Although the role of phosphor grain size (mean diameter of phosphorparticles) is not completely understood, weight fractions may changedepending on a particular grain size. Preferably, grain sizes are lessthan 15 μm, and more preferably, less than 12 μm, to avoid clogging ofdevices which dispose the phosphors. In one embodiment, the grain sizeof each phosphor type varies. In certain specific embodiments, the grainsize of

The phosphor is less than about 10 μm. Other devices, however, can beprepared with larger grain sizes.

Although unabsorbed light emitted from the LED contributes to colorrendering, unabsorbed light can sometimes escape without mixing withlight emitted from the phosphors, resulting in a reduced overallefficiency of the device. Thus, in one embodiment, the LED andcomposition are positioned within a reflector cup. A reflector cup canbe any depression or recess prepared from a reflecting material. Bypositioning the LED and phosphor particles in a reflector cup,unabsorbed/unmixed LED-emitted light can be reflected either back to thephosphor particles to eventually be absorbed, or mixed with lightemitted from the phosphors.

The function and advantage of these and other embodiments of the presentinvention will be more fully understood from the example below. Thefollowing example are intended to illustrate the benefits of the presentinvention, but do not exemplify the full scope of the invention.

EXAMPLE

For preparation of Sr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04) the starting materials59.41 g (566.3 mmol) Sr_(0.98)Eu_(0.02)O, 27.18 g (663.1 mmol) AlN and79.43 g (566.1 mmol) Si₃N₄ are thoroughly dry milled in an agate mortar.Afterwards the homogenized powder is annealed for 4 h at 1500° C. undernitrogen/hydrogen, whereby the crucible comprising the powder is putinto a second, charcoal-containing crucible.

After an intermittent milling step, the powder is annealed for 4 h at1600° C. under nitrogen/hydrogen again.

The resulting powder is milled on a roller bench for several hours. Themilled powder has an average particle size of 3-5 μm. Its quantumefficiency is 90% and its lumen equivalent is 1901 m/W. The color pointis at x=0.64, y =0.36.

For manufacturing a white illumination system based on a 460 nm emittingInGaN LED a phosphor blend comprising SrGa2S4:Eu andSr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04) is suspended into a silicone precursor. Adroplet of this suspension is deposited onto the LED Chip andsubsequently polymerized. A plastic lens seals the LED.

For manufacturing a white illumination system based on a 460 nm emittingInGaN LED a phosphor blend comprising (Ba,Sr)2SiO4:Eu andSr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04) is suspended into a silicone precursor. Adroplet of this suspension is deposited onto the LED Chip andsubsequently polymerized. A plastic lens seals the LED.

1. An illumination system, comprising a radiation source and afluorescent material comprising at least one phosphor capable ofabsorbing a part of light emitted by the radiation source and emittingvisible light of wavelength different from that of the absorbed light;wherein said at least one phosphor is an oxido-nitrido-silicate ofgeneral formulaEA_(2-z)Si_(5-a)B_(a)N_(8-a)O_(a):Ln_(z), wherein 0<z≦1 and 0<a<5,comprising at least one element EA selected from the group consisting ofMg, Ca, Ba and Zn and at least one element B selected from the groupconsisting of Ga and In, and being activated by a lanthanide selectedfrom the group consisting of cerium, europium, terbium, praseodymium andmixtures thereof.
 2. The illumination system according to claim 1,wherein the fluorescent material comprises a red phosphor having ageneral formula of EA_(2-z)Si_(5-a)B_(a)N_(8-a)O_(a):Ln_(z), wherein0<z≦1 and 0<a<5 and a green or yellow phosphor.
 3. The illuminationsystem according to claim 1, further comprising a green or yellowphosphor selected from the group of MS:Eu, Ce, Cu comprising at leastone element selected from the group M=Mg, Ca, Sr, and Zn; MN₂S₄:Eu, Cecomprising of at least one element selected from the group M=Mg, Ca, Sr,and Zn at least one element selected from the group N=Al, Ga, In, Y, La,Gd, (Re_(l-r)Sm_(r))₃(Al_(1-s)Gas)₅O_(l2):Ce, where 0≦r<1 and 0≦s≦1 andRe selected from Y, Lu, Sc, La and Gd and(Ba_(l-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and 0<z<1.4. The illumination system according to claim 1, wherein said radiationsource comprises a nitride compound semiconductor represented by thegeneral formula In_(i)Ga_(j)Al_(k)N, where 0≦i≦1, 0≦j≦1, 0≦k≦1 andi+j+k=1.
 5. The illumination system according to claim 1, wherein thesystem is a traffic sign.
 6. A phosphor capable of absorbing a part oflight emitted by the radiation source and emitting visible light ofwavelength different from that of the absorbed light; wherein said atleast one phosphor is an oxido-nitrido-silicate of general formulaEA_(2-z)Si_(5-a)B_(a)N_(8-a)O_(a):Ln_(z), wherein 0<z≦1 and 0<a<5comprising at least one element EA selected from the group consisting ofMg, Ca, Ba and Zn and at least one element B selected from the groupconsisting of Ga and In, and being activated with a lanthanide selectedfrom the group consisting of cerium, europium, terbium and mixturesthereof.
 7. A phosphor capable of absorbing a part of light emitted bythe radiation source and emitting light of wavelength different fromthat of the absorbed light; wherein said at least one phosphor is ofgeneral formulaSr_(1.96)Si₃Al₂N₆O₂:Eu_(0.04).
 8. The phosphor according to claim 6,wherein silicon is substituted by germanium.
 9. An illumination systemcomprising a radiation source and a fluorescent material comprising atleast one phosphor capable of absorbing a part of light emitted by theradiation source and emitting visible light of wavelength different fromthat of the absorbed light; wherein said at least one phosphor is anoxido-nitrido-silicate of general formulaEA_(2-z)Si_(5-a)B_(a)N_(8-a)O_(a):Ln_(z), wherein 0<z≦1 and 0<a<5comprising at least one element EA selected from a group of Mg and Znand at least one element B selected from a group of Ga and In, and beingactivated by a lanthanide selected from a group of cerium, terbium,praseodymium and mixtures thereof.
 10. The illumination system of claim9, wherein the fluorescent material comprises a red phosphor having ageneral formula of EA_(2-z)Si_(5-a)B_(a)N_(8-a)O_(a):Ln_(z), wherein0<z≦1 and 0<a<5 and a green or yellow phosphor.
 11. The illuminationsystem of claim 10, wherein the green or yellow phosphor is selectedfrom the group of MS:Eu, Ce, Cu comprising at least one element selectedfrom a group M=Mg, Ca, Sr, and Zn; MN₂S₄:Eu, Ce comprising of at leastone element selected from a group M=Mg, Ca, Sr, and Zn at least oneelement selected from a group N=Al, Ga, In, Y, La, Gd,(Re_(l-r)Sm_(r))₃(Al_(1-s)Gas)₅O_(l2):Ce, where 0≦r<1 and 0≦s≦1 and Reselected from Y, Lu, Sc, La and Gd and(Ba_(l-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and 0<z<1.12. The illumination system of claim 9, wherein the radiation sourcecomprises a nitride compound semiconductor represented by the generalformula In_(i)Ga_(j)Al_(k)N, where 0≦i≦1, 0≦j≦1, 0≦k≦1 and i+j+k=1. 13.The illumination system of claim 9, wherein the system is a trafficsign.